The uptake of nucleosides by mammalian cells is complex and mediated by multiple transport proteins. As data increases on the properties of these transporters it is becoming increasingly clear that there are differences in their substrate specificity and sensitivity to inhibitors. Furthermore, there are differences in the distribution of the transporters between some tumors and normal tissues that might be exploited in chemotherapy. One of the most striking is the presence of Na+-dependent, concentrative transport in several normal tissues, but not in most tumor cells. Since Na+- dependent nucleoside transporters are not inhibited by NBMPR or dipyridamole (inhibitors of the two equilibrative transporters found in most tumor cells), there is potential for the modulation of antimetabolite activity with these inhibitors. Aim 1 of this project will further define the nucleoside transport activities present in critical normal tissues such as intestine and bone marrow, and changes that may occur in nucleoside transport during differentiation of intestinal cells. The second aim of this project will test the hypothesis that highly selective therapy can be developed for some, but not all, tumors using inhibitors of de novo pyrimidine biosynthesis in combination with transport inhibitors to block nucleoside salvage. This therapeutic approach is predicted to be effective against tumors with predominantly NBMPR-sensitive nucleoside transport, but not against tumors with significant levels of NBMPR-insensitive transport. The mouse L1210 leukemia, which has three of the four known nucleoside transporters, and sublines of L1210 that are deficient in one or more of the transporters will be used as a model. This will allow us to evaluate the role of each of the transporters in tumor response against a common background with respect to other factors that may affect drug activity. The third aim will test the hypothesis that selective therapy can be developed for tumors with NBMPR-insensitive nucleoside transport using a cytotoxic nucleoside analog in combination with a transport inhibitor to block uptake of the analog by normal tissues. This will again be done using L1210 tumor model to determine the role of each of the transporters in the success or failure of therapy. The last aim will apply these therapeutic approaches to human tumors that grow as xenografts in immune deprived mice and as cell lines in culture. The in vitro/in vivo model will again permit us to examine at the biochemical level reasons for success or failure of the therapeutic strategy.